Conventional DC sources comprise, for example, batteries or a number of solar panels, which are connected in series to form so-called strings, with the result that the string voltage Ust corresponds to the sum of the voltages (DC source voltages Ust1, Ust2) of the number of series-connected solar panels (DC sources). A string loaded by a consumer produces a string current (Ist) and a string power (Ppv). A loading apparatus for the string is also referred to as a tracker and is often implemented by an electronic step-up converter. The step-up converter (tracker) ideally sets the string current Ist such that the maximum energy can be generated by the string; this working point is referred to as the “maximum power point” (MPP). Such maximum power point step-up converters (MPP trackers) are designed to step-up the string voltage Ust such that the string voltage Ust results in a constant output voltage Udc of the step-up converter (also referred to as “booster”). As is known, the DC source voltages Ust1, Ust2 have a high degree of variance; the level of the DC source voltage Ust1, Ust2 is firstly dependent on the temperature of the solar panels and secondly on the density of the solar radiation—the lower the temperature of the cells and the higher the density of the radiation, the higher the voltage of individual solar cells and therefore of the DC source voltages Ust1, Ust2. The DC source voltages Ust1, Ust2 vary from −40% to +40% of their rated value during operation.
A DC-to-AC convertor connected downstream of the step-up converter converts the output voltage Udc at its input into an AC voltage suitable for power supply systems at its output with an rms value Uac. The DC-to-AC converter feeds the energy from the DC sources into the power supply system, provided that the output voltage Udc of the step-up converter fulfills the following conditionUdc>√{square root over (2)}*Uac=1.42*Uac; where Udc is the output voltage of the step-up converter, and Uac is the rms value of the system voltage of the power supply system.
If the energy emission of a DC source, for example a solar panel, is disregarded or if a DC source, for example battery, is partially discharged, step-up converters are required in order to raise the output voltage Udc to the required value in order that a feed into the power supply system via the DC-to-AC converter is still possible.
Design and mode of operation of conventional step-up converters:
A conventional step-up converter 2, as illustrated by way of example in FIG. 1, for stepping up the DC source voltage of a DC source 1 substantially comprises a storage inductor 2.4, an electronic switch 2.7, a diode 2.6 and an output capacitor 2.9. In this case, the electronic switch 2.7 is formed from a transistor with an anti-parallel diode. Examples of such step-up converters can be found under the designation “Aufwärtswandler” [step-up converters] on the German version of “www.wikipedia.org” or in a text entitled “Vorlesung Schaltnetzteile” [Switched mode power supplies lecture] by Prof. Schmidt-Walter, Darmstadt High School, Germany (http://schmidt-walter.eit.h-da.de/smps/smps.html).
The DC source voltage Ust of the DC source is lower than the output voltage Udc across the output capacitor 2.9. When the switch 2.7 is switched on, a current Ist flows into the storage inductor 2.4, the corresponding energy of the current Ist is stored in the storage inductor 2.4.
Then, the switch 2.7 opens and the stored energy in the storage inductor 2.4 is introduced into the output capacitor 2.9 via the diode 2.6; the DC current Idc is flowing. The output capacitor 2.9 is also referred to as a DC bus capacitor. The output current Idc is controlled by the switching-on and switching-off (modulation) of the switch (2.7) (pulse-width modulation PWM).
Design and mode of operation of conventional step-down converters:
The design of a step-down converter as shown in FIG. 2 can be found under the designation “Abwärtswandler” [step-down converters] at the German version of “www.wikipedia.org” or in a text entitled “Vorlesung Schaltnetzteile” [Switched mode power supplies lecture] by Prof. Schmidt-Walter, Darmstadt High School, Germany (http://schmidt-walter.eit.h-da.de/smps/smps.html).
In contrast to the previously described step-up converter, known step-down converters 20 have, on the input side, a DC source 1 and, in parallel with this, a switch 2.7 in series with a diode 2.6; a storage inductor 2.4 and an output capacitor 2.9 are arranged in series, in parallel with this diode 2.6. The DC source voltage Ust of the DC source of the step-down converter 20 is greater than the output voltage Udc across the capacitor 2.9. When the switch 2.7 is switched on, a current flows into the storage inductor 2.4, and the energy corresponding to this current is stored in the storage inductor. Then, the switch 2.7 opens and the stored energy is introduced into the capacitor 2.9 via the diode 2.6.
Design and Mode of Operation of Conventional Bidirectional DC-to-DC Converters
By virtue of the combination of the circuits of a step-up converter (FIG. 1) and a step-down converter (FIG. 2), the design of known bidirectional DC-to-DC converters as shown in FIG. 3 is provided. Such DC-to-DC converters comprise a first capacitor 2.2 and a second capacitor 2.9, which, depending on the direction of energy flow, act as DC source and output capacitor or as output capacitor and DC source. A first switch 2.60 and a second switch 2.70 are connected in series and are arranged in parallel with the second capacitor 2.9. A series circuit comprising a storage inductor 2.4 and the first capacitor 2.2 is arranged in parallel with the first switch 2.60. The two switches 2.60 and 2.70 are constructed from transistors with anti-parallel diodes. As a result, the bidirectional DC-to-DC converter can convert energy in both directions between the first and second capacitors 2.2 and 2.9. In the case of such DC-to-DC converters, it can be considered disadvantageous that the total energy is converted by the switches 2.60 and 2.70 and the storage inductor 2.4 needs to store the total energy during the conversion.
The literature discloses many variants of such DC-to-DC converters which all have the common feature that the total energy is converted via switches and storage inductors: thus, DE 10 2004 037 446 B4 (transformerless inverter for solar system feed) provides a representative prior art with respect to step-up converters. In this case, a symmetrical DC-to-DC converter is shown which is similar to that in FIG. 3 and which nevertheless converts the total DC source voltage. The output capacitor 2.9 is in this case divided into two series-connected output capacitor elements, the central point of which is grounded.
WO 2006/011071 (3-phase Solar Converter Circuit and Method) contains a step-up converter with a conventional design for the total DC source power, wherein in this case the emphasis is on the activation of the line converter.
EP 2 148 419 A2 (Power converter arrangement for solar power installations and actuation method therefor) describes a symmetrical step-up converter (similar to that in DE 10 2004 037 446 B4). KR 2009 0128973 is concerned with further different circuit arrangements for step-up converters.
DE 10 2008 059330 (Compact three-phase inverter with upstream integrated step-up converter) describes an inverter unit with input step-up converters for solar applications. The step-up converter has a conventional design; the actual subject matter of this document consists in different configurations for the inverter.
Document US 2010 133904 (DC bus voltage control for two stage solar converter) comprises the entire arrangement of step-up converter with downstream DC-to-AC converter, wherein the step-up converter likewise converts the total power.
All of the step-up converters proposed here have the common feature that they can comprise large and complex circuits, have an undesirably high amount of energy losses, namely approximately 2% of the solar energy in such step-up converters is lost, wherein these energy losses which occur in the form of heat also need to be dissipated via corresponding cooling systems.
EP 2 173 024 A2 proposes an arrangement for the unipolar stepping-up of the DC source voltage from two DC sources using one step-up converter, as shown by way of example in FIG. 4, wherein this step-up converter requires two symmetrical storage inductors 2.4 and 2.5 and two diodes 2.6 and a unipolar switch 2.71. A first DC source 1.1 and a second DC source 1.2 are connected in series via a switch 2.71. The switch 2.71 is connected on both sides to the two diodes 2.6 to form a DC output 3.1, 3.2, wherein this DC output is supported by an output capacitor 2.9.
The current which flows when switch 2.71 is switched on is unipolar and is stored in the storage inductors 2.4, 2.5. When the switch 2.71 opens, the current stored in the storage inductors passes via the two diodes 2.6 to the DC output; in addition, a unipolar direct current flows parallel from the DC sources 1.1, 1.2 directly to the DC output 3.1, 3.2. The switch 2.71, the diodes 2.6 and the storage inductors 2.4, 2.5 thus only conduct part of the total direct current, the other part passes directly to the DC output. Therefore, up to 50% of the losses can be saved, with the result that the storage inductors 2.4, 2.5, the diodes 2.6 and the switch 2.71 can have smaller dimensions both electrically and also physically. On the other hand, harmonics, electromagnetic radiation, and the limited voltage range during operation as a step-up converter can be considered to be disadvantageous.
When the switch 2.71 switches, high-frequency AC voltages with the amplitude of the DC voltage arise. When the DC sources comprise solar panels, parasitic voltages with amplitudes of 1000 V and frequencies in the kHz range can occur, which cannot be withstood by most solar panels. Since the feed lines between the DC sources 1.1, 1.2 and the step-up converter 2 can be long, in this case unfiltered high-frequency currents and voltages in these feed lines result in undesired electromagnetic interference (EMC) in the surrounding environment. In addition, this step-up converter can only function when the sum of the two DC source voltages Ust1 and Ust2 is greater than or equal to the output voltage Udc. If the current flow reverses in the case of a relatively low total DC source voltage, the diodes 2.6 cannot take over the current, and the conversion stops. In addition, the voltage across the switch 2.71 is in this case likewise reversed. In this operating case, the switch 2.71 cannot be controlled; it is either switched on permanently or switched off permanently.
If a plurality of DC sources in the form of solar panels or series-connected solar panels (strings) are connected in parallel to form a so-called multi-string, the same output voltage Udc will be imposed on all of the solar panels or strings. During operation of a multi-string, however, shadowing or failure of individual solar panels can result in the entire affected multi-string being shut down, i.e. significant power failures result. It is even possible for energy from adjacent solar panels to be fed back to a shadowed solar panel. By shadowing and by aging of individual solar panels, the working points “maximum power points” (MPP) of the individual solar panels already mentioned above can deviate from one another. These deviations cannot be taken into consideration individually in the case of multi-strings, however, since no regulation of the power of individual solar panels is provided. A solution to this problem would be to assign a step-up converter to each individual solar panel or string, which appears to be impracticable for reasons of cost. In addition, the step-up converters have up to 1.5% of the rated power as losses, which means that a large amount of solar energy is consumed and, in addition, complex temperature management is required. One example of such a circuit is disclosed in DE 101 36 147 B4 (photovoltaic AC generator).
JP 9261949 (DC/DC Converter and Solar Generation System) deals with the problem that multi-strings can have different numbers of solar panels, or solar panels become ineffective as a result of shadowing, such that a “further” step-up converter is assigned to the multi-string with fewer solar panels, said “further” step-up converter matching the output voltage Udc correspondingly. With this arrangement, it is not possible to respond flexibly to temporarily different voltage ratios within a multi-string unless the “further” step-up converter is provided in all solar panels, which would increase the complexity correspondingly to an undesirable extent.